Utility:
21

Introduction

First thing you have to know is about me : English isn't my native language, so I'm sorry for the mistakes you can find in this build. (If it happen, send me a PM ! Thanks !)

Ok, let's go !

Cassiopeia, The Serpent's Embrace.

PRO-Insane harass capability-Good control with her W (Low CD)-Great mobility-When you got a good map control, can do insane anticipated action through the bushes / jungle, forcing your opponent to back, or kill him, without taking risk.-Sexy. Seriously, have you ever seen a woman with a tail of that length ?

CON-Squishy.-No real escape. Very dependent of flash for that.-Hard to play and master. Seriously. Need an insane capacity to anticipate. Not only for her skills.-Totally useless if you can't play her correctly.

Runes

Ok, let's talk about Runes. (I'll not speak about masteries.)

9x Greater Mark of Magic Penetration for the magic penetration. Build is based on that, and give a very good bonus to your damage Early game. (And even after if they don't take MR)

2x Greater Quintessence of Movement Speed you'll need more mobility for your skill, and gank the other lane. You will get like ~329 move at the beginning without boots, and 400+ with sorcerer's shoes.

About your Skills

Deadly Cadence : Your passive, and why I don't recommend Tear of the Goddess. This item is good for champions who consume a lot of mana, but here, with your runes and the Doran's Ring, it's useless, your mana regen is enough. Personally, I love this passive, because you can stay longer on your lane and don't lose XP with a back for your mana. If you aim correctly and don't spam your poisons for the fun, you can stay without back for years. Yep sir.

Noxious Blast : Your skill-shot. Like Lay Waste (Karthus), but with a smaller range. When you hit the key, the spell goes where is your mouse, no need to click on the location. It's your "Principal" harass skill. Work in synergy with Twin Fang : Poison on the enemy, spam Twin Fang. If you opponent is playing safe, it's more difficult to touch him with the poison, be carefull ! The most common reaction you will have is spamming Noxious Blast, without take care of your mana. Stay calm, and don't worry if it happen : He's scared. He KNOW you can make him back just with that.

Miasma : Slow, hallelujah ! It make a slowing poison zone where you use it, who extend with time (On 7 seconds). It works with Twin Fang too ! You slow them, use Noxious Blast and spam that, that's simple =p Ho, and the most important thing you have to know on that skill : It give the vision of the area. Yes, you can spot a lot of things with that. And NOBODY can gank you from bushes. You HAVE TO use it every time you don't know if they are here. You can spot dragon or mid jungle (What? You haven't warded it ?) through the wall too !

Twin Fang : Ok. Now, silence. Thanks god for giving us this skill. THE skill. Your opponent will hate you every time you will use that. If they are squishy, they will insulte you, rage, report you for nothing, and then suicide.
...
Ok, maybe I exaggerate. But this skill rocks ! Pretty cool damage, not a skill-shot (You have to target the enemy), and the best : If your target is poisoned, the CD become 0.5 instead of 5. Yep. It mean spamming. And when you face something like a Teemo, and you spam it on him... That's not beautiful, at all.
Last thing you have to know is : USE IT ! Even if your target isn't poisoned. I don't mean "All the time", obviously, but if they have good reflex and dodge most of your Noxious Blast / Miasma, it's more simple to harass them a little with that.

Petrifying Gaze : For finish, your ultimate =D Very good range (Big cone !), good damages, control... Everything you'll need to win your team fight. The first rule to use it is simple : DON'T ENGAGE THE FIGHT. I know, that's obvious, if you do, you die. But I mean, if someone engage, don't jump into the fight. Wait for the best enemies placement, and then, ULTIMATE ! If you are doing this right, that's simple : You stun the enemy team. For 2 second. And make cool damage. That make the difference between "Teamfight" and "ACE!". You have an ultimate who can turn any teamfight in your advantage, use it properly !

Items

Now, Items !

First, for those wondering why there is ward everywhere in the build order, that's for those who alt-tab in game to see the guide : DON'T FORGET THE WARDS !

Ok, now, for the rest of the build, this is YOUR choice. You can go for an Abyssal Mask, for the magic penetration and magic resistance. (At this point, you'll get 15%+68.5 magic penetration. So someone with 100 MR will get 16.5 instead.)

If you are sadistic, or simply if your opponent go with a lot of MR, you can go for the Void Staff. And world is yours.

Your opponents got a lot of physical damage, and rape you? Zhonya's Hourglass is your friend. Kick-*** active effect, a few armor, and a lot of power.

You have to adapt your item build, according to your opponents build / play-style. These are just the items that works for me when I use them.

Ho, and don't forget to buy the god-mod : Rabadon's Deathcap. "Hi. Poison. You have to back. Thanks for playing."

Summary

This build is made for 3v3. It can work too in 5v5, obviously, but may be less efficient. There is 2 important things you'll need :

Good map control

Green circle are the ward you MUST HAVE. You can't say "We've got the map control" without controlling these 2 zones. After that, you have the minimum required to make gank.Red circle are the good spot for warding when you already have control of the green zone. If you can ward here, do it. Your opponent will make the neutral minions when they can, and it's always cool to know when. Don't forget Cassiopeia can make insane harass, and a simple farming for them can turn into back if you're doing things right.Yellow circle are the optional ward. If you don't control these spot, it's not a problem. The bot one is principally for your allies at early / mid game. The top one is for preventing enemies to do the jungle buff.

I know, it's a loot of spot, and so a lot of wards. But if each player of the team buy one ward when they back... :3

Good to know : Miasma can spot bushes, or mid jungle through the top lane, for example. Use it !

-A good teamplay. It mean communication, synchronizing your action with your allies, and NO SOLO. You are not an hero, even if you are feed.